Download Glaims (http://www.macupdate.com/info.php/id/27708/glims) 

Quit Safari
 
Run the Glaims installer
 
Restart Safari
 
Open the Safari preferences and click the "Glims", click "Search Engines" and "add" a new entry
 
Name it "Bing" and enter the Query URL "http://www.bing.com/search?q=#query#"
 
Click Add/Set

Close Safari preferences
 
Change the search engine in Safari by clicking on Google icon in the search box and select Bing
 
Enjoy!